The present invention relates to a polarizing plate, particularly to having high durability and high polarization efficiency that is useful for liquid crystal displays (LCDs) and more specifically, to an improvement in a polarizing plate for substantially preventing mainly drop in polarization efficiency, hue shift, and light leakage even under a high humidity/high temperature environment.
A conventional polarizing plate with high polarization efficiency is generally such that a cellulose-triacetate-based (hereinafter, referred to as TAC) film, which serves as a protective film, is laminated on a polarizing film in which iodine or a dichroic dye is adsorbed and oriented in a polyvinyl-alcohol-based (hereinafter, referred to as PVA) film with an aqueous solution of PVA resin, which serves as an adhesive, in a state of wet or semidry flowability.
However, since the water absorption and water vapor permeability of TAC is high in a polarizing plate using TAC for the protective film, deterioration in polarization performance under a high humidity/high temperature environment, specifically, drop in polarization efficiency, hue shift, and light leakage under crossed nicols, has been significant.
In order to overcome these problems, polarizing plates that use a film made of a resin with low water absorption and low water vapor permeability for the protective film have been proposed.
For example, Japanese Unexamined Patent Publication No. 7-77608 discloses a polarizing plate such that a film serving as a protective film and made of a thermoplastic saturated norbornene-based resin is adhered to a PVA-based polarizing film using an acrylic-based adhesive or a polyester-isocyanate-based adhesive. After such a polarizing plate is subjected to an environment of 80xc2x0 C. and 90%RH for 500 hours, the polarization efficiency is 95% or higher and the single transmissivity is 38% or higher.
In addition, Japanese Unexamined Patent Publication 7-294732 discloses a polarizing plate such that a film having a photoelastic coefficient of 25.0xc3x9710xe2x88x9213 cm2/dyne or less, for example, a film made of an amorphous polyolefin as Zeonex, or polymethyl methacrylate such, serves as a support for a polarizing element film, and the support is adhered to the polarizing element film using an acrylic-based adhesive. After such a polarizing plate is subjected to an environment of 60xc2x0 C. and 90%RH for 100 hours, the pyschometric lightness is small.
However, although these polarizing plates are able to suppress a drop in polarization efficiency under a wet heat environment, it cannot be said that suppression of hue shift and light leakage is sufficiently realized.
In the view of the foregoing and other problems, it is an object of the present invention to provide a polarizing plate with which, not only suppression of a drop in polarization efficiency is achieved, but also with which hue shift and light leakage do not substantially arise, under a high temperature/high humidity environment.
A polarizing plate of the present invention comprises a polyvinyl-alcohol-based polarizing film, a protective film, mainly composed of a cyclic-olefin-based resin having thereon two anchor coat agent layers, laminated on at least one side of the polarizing film with an adhesive.
The amount of change in optical in-plane retardation of the protective film is 5 nm or less after 24 hours in an atmosphere of 80xc2x0 C., and the wetting tension of the laminated surface of the protective film is 500 xcexcN/cm (23xc2x0 C.) or more.
A first anchor coat agent layer is made of polyisocyanate and polyester polyol and/or polyether polyol. A second anchor coat agent layer is made of polyvinyl alcohol.
The adhesive is made of polyvinyl alcohol.
The polarizing film of the present invention is produced by uniaxially stretching and orienting a film made of PVA or a derivative thereof, and subsequently, adsorbing iodine, carrying out a boric acid solution treatment, and drying the film while under tension. Such a film also may be produced by immersing a film made of PVA or a derivative thereof in an aqueous solution of iodine such that the iodine is adsorbed, and subsequently, uniaxially stretching and orienting the film in a boric acid solution and drying the film while under tension. Polarizing films that utilize dichroic dyes, such as those that are azo-based, anthraquinone-based, and tetrazine-based, instead of iodine are fabricated in the same manner as well.
The polarization efficiency of a polarizing film obtained in such a manner is preferably, 95.0% or higher, more preferably, 99.0% or higher, and even more preferably, 99.7% or higher.
Hue shift as referred to in the present invention denotes a phenomenon such that when a single polarizing plate or crossed nicols is placed in a wet heat atmosphere, hue shift occurs with the single polarizing plate or the crossed nicols.
When a liquid crystal display employing polarizing plates with which hue shift arises is used for a long period, the hue of the display changes and contrast deteriorates, becoming one cause of deterioration in the performance of the liquid crystal display.
Light leakage as referred to in the present invention denotes a phenomenon such that in-plane luminance changes when two polarizing plates arranged to have a crossed nicols relation are placed in a wet heat environment.
When a liquid crystal display that employs polarizing plates that generate light leakage is used for a long period, light leaks at the edges of the display when black is displayed, and thereby display contrast deteriorates, becoming one cause of deterioration in the performance of a liquid crystal display.
Having fully considered how to provide a polarizing plate with which, not only suppression of a drop in polarization efficiency is achieved, but also with which hue shift and light leakage do not substantially arise, under a high temperature/high humidity environment, the present inventors came to the following conclusion, by which the present invention was achieved.
First, suppression of a drop in polarization efficiency under a high temperature/high humidity environment can be achieved by using a film with low water absorption and low water vapor permeability for the protective film of a polarizing plate. Suppression of hue shift under a high temperature/high humidity environment can be realized by sufficiently adhering a polarizing film and a protective film and by suppressing reversion in the alignment of the polarizing film. Suppression of light leakage under a high temperature/high humidity environment can be realized by using a film having a small amount of change in optical in-plane retardation for the protective film of a polarizing plate.
The present inventors then fully considered how to substantiate these inferences.
For the present invention, a film mainly composed of cyclic-olefin-based resin was employed for the protective film of the polarizing plate, because such a film has low water absorption and low water vapor permeability, and various physical properties required of a protective film for a polarizing plate, such as light transmissivity. (In addition, because cyclic-olefin-based resin has a small photoelastic coefficient, it was conjectured to be useful in preventing light leakage.)
In the present invention, cyclic-olefin-based resin is used as a general term, specific examples (a) to (d) being shown below.
(a) polymers that are ring-opening (co-)polymers of cyclic olefin with hydrogen added as needed
(b) (co-)polymers with cyclic olefin attached
(c) random copolymers of cyclic olefin and an xcex1-olefin such as ethylene or propylene
(d) graft modified substances that result when the above (a) to (c) are modified with unsaturated carboxylic acid or derivatives thereof.
The cyclic olefin is not particularly limited, examples including norbornene, tetracyclododecene, and derivatives thereof (for example, substances containing a carboxyl group or an ester group).
Known additives such as ultraviolet absorbers, organic or inorganic antiblocking agents, slip additives, antistatic agents, and stabilizers may be added appropriately to the cyclic-olefin-based resin.
The method of forming a protective film from cyclic-olefin-based resin is not particularly limited, it being possible to employ methods such as solution casting, extrusion, and calendering.
Examples for a solvent used in solution casting include alicyclic hydrocarbons such as cyclohexane and cyclohexene and derivatives thereof, as well as aromatic hydrocarbons such as toluene, xylene, and ethyl benzene and derivatives thereof.
The thickness of the protective film is commonly 5-150 xcexcm, preferably 10-100 xcexcm, and more preferably 20-60 xcexcm. When thickness is too thin, a film tends to be difficult to handle and when thickness is too thick, the amount of change in optical in-plane retardation tends to be large.
In order to determine the relationship between hue shift and the adhesive strength of a protective film/polarizing film under a high temperature/high humidity environment, tests were carried out using various adhesives with varying adhesive strength, and as was initially predicted, it was determined that there is a correlation between hue shift and the adhesive strength of a protective film/polarizing film. However, there was no adhesive with which hue shift substantially did not occur. In consideration of this, the present inventors considered various ways of increasing the adhesive strength and suppressing hue shift to the greatest possible degree and eventually, discovered two ways of overcoming the problems of adhesive strength and hue shift.
Specifically, first, the wetting tension of the surface of the protective film to be anchor coated is made to be 500 xcexcN/cm (23xc2x0 C.) or higher and preferably 550 xcexcN/cm (23xc2x0 C.). In order to achieve this value, it is not necessary to employ a particular technique, it being possible to use a technique known in the art. Examples for a surface treatment include a corona discharge treatment, an ultraviolet irradiation treatment, and a chemical treatment. The corona discharge treatment or ultraviolet irradiation treatment may be carried out in air or in an atmosphere of nitrogen or a rare gas.
When the wetting tension is lower than 500 xcexcN/cm (23xc2x0 C.), sufficient adhesive strength cannot be obtained.
Secondly, an anchor coat agent made of polyisocyanate and polyester polyol and/or polyether polyol is coated, as a first layer, on the protective film surface and dried, then on the resulting film, an anchor coat agent made of polyvinyl alcohol is coated, as a second layer and dried, and subsequently, an adhesive solution of polyvinyl alcohol is adhered to the polarizing film in a wet or semidry state.
The coating and drying of the anchor coat agents for two layers may be carried out directly before adhering the protective film and the polarizing film with an adhesive solution, or the film may be wound temporarily after coating the protective film surface with the anchor coat agents for two layers and drying, and adhering of protective film and polarizing film with an adhesive solution carried out at a later time.
The polyisocyanate of the anchor coat agent for the first anchor coat agent layer has two or more isocyanate groups in each molecule, the polyester polyol of the anchor coat agent for the first layer has ester bonds in its molecules and two or more hydroxyl groups in each molecule, and the polyether polyol of the anchor coat agent for the first layer has ether bonds in its molecules and two or more hydroxyl groups in each molecule.
The skelton structure of the polyisocyanate may be an aromatic ring or another structure, a long chain alkylene group being preferable from the perspective of adhesive strength. This is thought to be because long-chain alkylene has a degree of flexibility and thus good adhesion with the protective film surface is expected.
The mixing ratio of polyisocyanate and polyester polyol and/or polyether polyol is preferably, 20:1-1:20 and more preferably, 5:1-1:5, in consideration of the equivalence weight ratio of the hydroxyl groups and the isocyanate groups.
It is preferable that the amount of anchor coat agent be such that a thickness after drying of 0.001-5 xcexcm results and more preferable that a thickness of 0.01-2 xcexcm results. When too little anchor coat agent is used, adhesive strength often cannot be realized to the degree desired, and when too much is used, coating nonuniformities easily arise, which is often undesirable in terms of hue shift and light leakage.
Note that it cannot be said that substances that react with polyisocyanate other than polyester polyol and/or polyether polyol mentioned above, for example, acrylic-based substances, sufficiently demonstrate advantageous effects in terms of suppressing hue shift due to there weak adhesive strength to the protective film.
The polyvinyl alcohol of the anchor coat agent for the second layer is mainly composed of a resin that is obtained by carrying out a saponification treatment on vinyl acetate resin. It is preferable that the degree of polymerization be 1000-3000 and that the degree of saponification be 85% or higher and more preferable that the degree of polymerization be 1500-3000 and the degree of saponification be 98% or higher. Other monomers such as monomers copolymerized appropriately with a small amount of acrylic acid, crutonic acid, itaconic acid, and the like or monomers modified by alkyl groups, epoxy groups, or the like may be used.
A substance that reacts with polyvinyl alcohol, such as polyisocyanate, boric acid, alkylene diamine, and epoxy resin, may be added to the polyvinyl alcohol. Advantageous effects are obtained particularly with polyisocyanate in terms of improving in water resistance and easy handling.
The advantageous effects of the second anchor coat layer are as follows. First, the adhesive strength does not deteriorate for an extended period of time. When there is only the first anchor coat agent layer present, because the layer contains isocyanates, the layer easily deteriorates, and thus when only the first anchor coat is provided on the protective film, and stored for an extended period of time and then adhered to the polarizing plate, the adhesive strength is significantly reduced. However, when the second anchor coat layer is provided, an advantageous effect that the adhesive strength is not reduced for an extended period of time is obtained. Accordingly, a protective film having thereon the first anchor coat layer and the second anchor coat layer can be stored for an extended period of time, allowing the production and storing of the protective films in advance. Secondly, the second anchor coat layer has a degree of water absorbing property, and therefore an advantageous effect is obtained that the layer speedily absorbs moisture in an adhesive solution when adhered to the polarizing plate to complete adhesion in a short time.
It is preferable that the amount of anchor coat agent be such that a thickness after drying of 0.01-30 xcexcm results, more preferable that a thickness of 0.1-15 xcexcm results, and even more preferable that a thickness of 0.5-5 xcexcm results. When too little anchor coat agent is used, adhesive strength to the first anchor coat layer often cannot be realized to the degree desired, and when too much is used, the cost effectiveness decreases.
The polyvinyl alcohol of the adhesive is mainly composed of a resin that is obtained by carrying out a saponification treatment on vinyl acetate resin. It is preferable that the degree of polymerization be 1000-3000 and that the degree of saponification be 94% or higher, and more preferable that the degree of polymerization be 1500-3000 and the degree of saponification be 98% or higher. Other monomers such as monomers copolymerized appropriately with a small amount of acrylic acid, crutonic acid, itaconic acid, and the like or monomers modified by alkyl groups, epoxy groups, or the like may be used.
It is preferable that the amount of adhesive solution be such that a thickness after drying of 0.01-10 xcexcm results, more preferable that a thickness of 0.02-5 xcexcm results, and even more preferable that a thickness of 0.05-3 xcexcm results. When too little adhesive is used, adhesive strength often cannot be realized to the degree desired, and when too much is used, the cost effectiveness decreases.
A substance that induces reactive curing with polyvinyl alcohol, such as polyisocyanate, boric acid, alkylene diamine, and epoxy resin, may be added.
In order to determine the relationship between light leakage and the amount of change in optical in-plane retardation under a high temperature/high humidity environment, 50 xcexcm thick protective films having varying amounts of change in optical in-plane retardation and mainly composed of various cyclic-olefin-based resins, and polarizing plates were fabricated using these protective films. Investigation into the amount of light leakage using a method described later revealed that there is a correlation between light leakage and the amount of change in optical in-plane retardation, as was initially predicted, and it was discovered that light leakage substantially does not occur when the amount of change in optical in-plane retardation is 5 nm or less.
In the tests, substances described previously were used for the laminated surface of protective film, the anchor coat agents, and the adhesive.
The amount of change in optical in-plane retardation was obtained as follows. As shown in FIG. 2(a), a protective film 3 cut to a size lengthxc3x97width=100 mmxc3x97100 mm was attached to a glass substrate 1 with a binder 2 made of acrylester-based base resin and an isocyanate-based curing agent interposed. The optical in-plane retardation was measured in each of nine sections divided as shown in FIG. 2(b), and the average value R0 was obtained. After then subjecting this to an 80xc2x0 C. atmosphere for 24 hours, the optical in-plane retardation was measured in the same nine sections, and the average value R was obtained. The difference between R and R0 (Rxe2x88x92R0) was taken to be the amount of change in optical in-plane retardation. Note that an arrow shown in FIG. 2(b) indicates the length of the protective film 3.
For the most part, the amount of change in optical in-plane retardation is dependent on distortion of molecular chains in the protective film and on residual shrinkage percentage.
When a protective film is fabricated by solution casting, distortions in the molecular chains arise in the drying step. In addition, residual shrinkage percentage is affected by the orientation of the cyclic-olefin-based resin when the solution is stretched on a metal drum or an endless belt, by the orientation of cyclic-olefin-based resin caused by pulling tension in the drying step, and by the residual solvent.
When a protective film is fabricated by extrusion, distortions in the molecular chains arise during cooling and hardening with a chill roll after extrusion from an extruder. In addition, residual shrinkage percentage is affected by the draw during extrusion from the extruder and by the orientation of the cyclic-olefin-based resin caused by pulling tension from the point of cooling and hardening to the point of winding.
In order to make the amount of change in optical in-plane retardation of the protective film 5 nm or less, it is necessary to employ suitable methods such as correcting distortions in molecular chains in the protective film and reducing the residual shrinkage percentage.
For example, methods of correcting distortions of molecular chains and of reducing the residual shrinkage percentage include heating the film under a minus draw before winding the film and leaving the loosely wound film in a heat chamber. In addition, in the case of employing solution casting for production, leaving the film in a drying oven for a long period is one method of reducing the residual solvent, preferably until none remains. Adding preferably 0.1-20% by weight, more preferably 0.5-10% by weight, and even more preferably 0.5-5% by weight with respect to resin of a plasticizer such as dioctyl adipate, dioctyl phthalate, or isodecyl adipate to the casting solution before hand, is another method. Because the drying time required for practically eliminating the residual solvent is reduced by ⅕-{fraction (1/20)} when a plasticizer is added, such a method is advantageous from the perspective of productivity and cost of equipment. The advantageous effects of adding a plasticizer are conjectured to be as follows. That is, it is thought that because cyclic-olefin-based resin molecules have a bulky skelton structure, solvent that enters into these gaps does not easily evaporate, but when a plasticizer is added, the plasticizer enters into the gaps so as to discharge the solvent from the gaps.
The residual shrinkage percentage necessary in order to make the amount of change in optical in-plane retardation of the protective film 5 nm or less is such that surface shrinkage percentage according to a measuring method described later is preferably 0.8% or less, more preferably 0.5% or less, and even more preferably, 0.3% or less.